The present invention relates to a method of burning a premixed gas and a combustor for practicing the method and, more particularly, to a gas turbine combustor and a burner for practicing the method.
Generally, NOx which is produced during combustion includes fuel NOx and thermal NOx with the former being produced from nitric compounds in the fuel and the latter from nitrogen in the air.
To reduce emission, fuel NOx is reduced to N.sub.2 and O.sub.2 by forming a reduction zone in the combustion zone, but essentially it is most effective to reduce nitrogen in the fuel, that is, to modify the quality of the fuel.
There are water injection method, gas recirculating burning method, fuel dilution combustion method or a like method for reducing thermal NOx. These methods achieve reduction of thermal NOx by reducing the temperature of the flame but they are liable to deteriorate stability of the flame.
As a known combustion method in a combustor, so called diffusion combustion has been widely adopted in which fuel and air were injected from respective nozzles but recently, premix combustion in which fuel and air are premixed and then injected from the same nozzle are being brought to use.
Premix combustion has the following two main advantages:
One advantage is that premix combustion reduces the reaction zone of combustion. That is, the gas ejected from the nozzle is a premixed gas consisting of fuel and air. This obviates any zone to produce a premixed gas downstream of the fuel nozzle as in the prior art, and hence it is possible to shorten the flame and to provide high load combustion.
The other advantage is that premix combustion is capable of reducing thermal NOx. In diffusion combustion in which fuel and air are injected into a combustion chamber from different nozzles, a zone of which air ratio (theoretical mixing ratio) is close to 1 is inevitably produced in the mixing process of the fuel and air in the combustion chamber event if the fuel is burned out in a diluted condition. Thus, it is well known that reduction of NOx is difficult in diffusion combustion. On the contrary, in fuel dilution premix combustion method in which fuel and excess air is premixed and then burned, the fuel is burned in a dilute combustion condition in the overall combustion zone, and hence it is easy to reduce NOx.
Such a dilution premix combustion method is adopted in a combustor of a gas turbine disclosed in Japanese Patent (examined) Publication NO. 62 (1987)-35016, for example.
Although dilution premix combustion reduces NOx at relatively low flame temperature due to combustion in excess air, it is inferior in stability of the premix flame.
To improve the stability of the premix flame, it is necessary to form the flame in the vicinity of a theoretical mixing ration but combustion in the vicinity of the theoretical mixing ratio produces a lot of NOx.
Thus, the condition to facilitate forming of stable flame and the condition to suppress production of NOx are in conflict with each other. This requires flame stabilization to form stable flame even in an excess air ratio condition or combustion technology which enables NOx to be reduced in combustion in the vicinity of a theoretical mixing ratio.
The known art of stabilizing premix flame includes combustors disclosed in U.S. Pat. Nos. 4,051,670 and 4,150,539, for example.
The combustor of the former patent is provided with a swirling mechanism to swirl a gas mixture of air and fuel in the combustion chamber, and the combustor further includes an pressure reducing mechanism to reducing pressure in a portion of the zone in which the swirl is formed. The ignition of the fuel is positively achieved by introducing a hot combustion gas into the swirl of the mixture gas, so that the flame is stabilized.
The combustor of the latter includes a baffle arranged at an exhaust nozzle of a mixture gas of air and fuel. A hot combustion gas formed downstream of the baffle serves as an ignition source and hence stabilizes the flame.
Various other attempts to stabilize flame have proposed as described in, for example: Japanese Patent (unexamined) Publication No. 59 (1984)-74,406 utilizing a pilot flame; and Japanese Patent (unexamined) Publication No. 64 (1989)-54,122 forming a swirl.
In these last-mentioned publications, only a small mixing zone to mix the combustion gas and the premixed gas is formed.
When dilution premix combustion is conducted according the flame stabilizing methods described above, both stabilization of premix flame and some reduction of NOx are achieved.
However, recently emission standards against NOx which causes photochemical smogs is becoming stricter, and hence it is desired to further reduce NOx.